How and why did a simple yeast evolve a complex life cycle?

The yeast Saccharomyces cerevisiae has long been domesticated by humans. It is an ideal organism for evolution experiments. It can grow rapidly to large population sizes, it has an interesting life cycle which includes both sexual and asexual stages, and it is extremely well understood as a laboratory model organism. But very little is known about its natural life. By studying S. paradoxus, a wild relative of S. cerevisiae, both in its natural environment and in the laboratory, we can understand more about how and why features of the life cycle such as sporulation, sex, and signalling evolved.